Hydrocarbons, such as oil and gas, may be recovered from various types of subsurface geological formations. The formations typically consist of a porous layer, such as limestone and sands, overlaid by a nonporous layer. Hydrocarbons cannot rise through the nonporous layer, and thus, the porous layer forms a reservoir in which hydrocarbons are able to collect. A well is drilled through the earth until the hydrocarbon bearing formation is reached. Hydrocarbons then are able to flow from the porous formation into the well.
In what is perhaps the most basic form of rotary drilling methods, a drill bit is attached to a series of pipe sections referred to as a drill string. The drill string is suspended from a derrick and rotated by a motor in the derrick. A drilling fluid or “mud” is pumped down the drill string, through the bit, and into the well bore. This fluid serves to lubricate the bit and carry cuttings from the drilling process back to the surface. As the drilling progresses downward, the drill string is extended by adding more pipe sections.
When the drill bit has reached the desired depth, larger diameter pipes, or casings, are placed in the well and cemented in place to prevent the sides of the borehole from caving in. Cement is introduced through a work string. As it flows out the bottom of the work string, fluids already in the well, so-called “returns,” are displaced up the annulus between the casing and the borehole and are collected at the surface.
Once the casing is cemented in place, it is perforated at the level of the oil bearing formation to create openings through which oil can enter the cased well. Production tubing, valves, and other equipment are installed in the well so that the hydrocarbons may flow in a controlled manner from the formation, into the cased well bore, and through the production tubing up to the surface for storage or transport.
This simplified drilling and completion process, however, is rarely possible in the real world. Hydrocarbon bearing formations may be quite deep or otherwise difficult to access. Thus, many wells today are drilled in stages. An initial section is drilled, cased, and cemented. Drilling then proceeds with a somewhat smaller well bore which is lined with somewhat smaller casings or “liners.” The liner is suspended from the original or “host” casing by an anchor or “hanger.” A seal also is typically established between the liner and the casing and, like the original casing, the liner is cemented in the well. That process then may be repeated to further extend the well and install additional liners. In essence, then, a modern oil well typically includes a number of tubes wholly or partially within other tubes.
Moreover, hydrocarbons are not always able to flow easily from a formation to a well. Some subsurface formations, such as sandstone, are very porous. Hydrocarbons are able to flow easily from the formation into a well. Other formations, however, such as shale rock, limestone, and coal beds, are only minimally porous. The formation may contain large quantities of hydrocarbons, but production through a conventional well may not be commercially practical because hydrocarbons flow though the formation and collect in the well at very low rates. The industry, therefore, relies on various techniques for improving the well and stimulating production from formations. In particular, various techniques are available for increasing production from formations which are relatively nonporous.
One technique involves drilling a well in a more or less horizontal direction, so that the borehole extends along a formation instead of passing through it. More of the formation is exposed to the borehole, and the average distance hydrocarbons must flow to reach the well is decreased. Another technique involves creating fractures in a formation which will allow hydrocarbons to flow more easily. Indeed, the combination of horizontal drilling and fracturing, or “frac'ing” or “fracking” as it is known in the industry, is presently the only commercially viable way of producing natural gas from the vast majority of North American gas reserves.
Formations are fractured most commonly by pumping a fluid, usually water, into the formation at high pressure and flow rates. The fluid will cause the formation to fracture and create flow paths to the well. Proppants, such as grains of sand, ceramic or other particulates, usually are added to the frac fluid and are carried into the fractures. The proppant serves to prevent fractures from closing when pumping is stopped.
A formation usually is fractured at various locations. The formation is rarely fractured all at once, but multiple locations within the wellbore may be fractured simultaneously. Especially in a typical horizontal well, the formation usually is fractured at a number of different locations or clusters of locations along the bore in a series of operations or stages. For example, an initial stage may fracture the formation near the bottom of a well. The frac job then would be completed by conducting additional fracturing stages in succession up the well, each stage fracturing a particular location or cluster of locations.
Fracturing typically involves installing a production liner in the portion of the well bore which passes through the hydrocarbon bearing formation. In shallow wells, the production liner may actually be the casing suspended from the well surface. In either event, the production liner is provided with openings at predetermined locations along its length. The openings will allow fluid to be diverted from the liner into the formation. They most commonly are provided by perforating the liner, i.e., forming holes through the liner, or by installing a series of valves in the liner.
Frac valves typically include a cylindrical housing that may be threaded into and forms a part of a production liner. The housing defines a central conduit through which frac fluids and other well fluids may flow. Ports are provided in the housing to allow fluid to flow out of the liner and into the formation. The ports may be opened by actuating a sliding sleeve. The sliding sleeves usually are actuated either by creating hydraulic pressure behind the sleeve itself or by dropping a ball on a ball seat which is connected to the sleeve. Hydraulic pressure then is built up behind the ball to actuate the sleeve. Typical multi-stage fracking systems will incorporate both types of valves.
Halliburton's RapidSuite sleeve system and Schlumberger's Falcon series sleeves, for example, utilize a hydraulically actuated “initiator” valve and a series of ball-drop valves. The production liner is provided with a hydraulically actuated sliding sleeve valve which, when the liner is run into the well, will be located near the bottom of the well bore in the first fracture zone. The production liner also includes a series of ball-drop valves which will be positioned in the various other fracture zones extending uphole from the first zone.
A frac job will be initiated by increasing fluid pressure in the production liner. The increasing pressure will actuate the sleeve in the bottom, hydraulic valve, opening the ports and allowing fluid to flow into the first fracture zone. Once the first zone is fractured, a ball is dropped into the well and allowed to land on the ball seat of the ball-drop valve immediately uphole of the first zone. The seated ball isolates the lower portion of the production liner and prevents the flow of additional frac fluid into the first zone. Continued pumping will shift the seat downward, along with the sliding sleeve, opening the ports and allowing fluid to flow into the second fracture zone. The process then is repeated with each ball-drop valve uphole from the second zone until all zones in the formation are fractured.
It will be appreciated that those systems are designed to fracture one zone at a time. Fracturing a single zone in each stage, other factors being equal, can allow for greater control over the process and will require less pumping capacity, especially when the formation is relatively hard and nonporous. On the other hand, for certain formations and well designs, operators may prefer to fracture multiple zones in a single stage. By fracturing clusters of zones in a single stage, the entire formation can be fractured more quickly.
When the well bore will be fractured in clusters, the production liner will incorporate a series of “static” valves, one for each cluster. Many static valves are ball-drop valves similar to the valves discussed above. They incorporate a ball seat that not only enables the valve to be opened, but once opened, allows a ball seated thereon to restrict the flow of fluid through the valve and into downhole zones that have already been fractured. Instead, fluid is forced out of the valve into the adjacent formation so that it may be fractured.
When a cluster of zones will be fractured in a single stage, the production liner also will incorporate one or more cluster valves uphole from each static valve. The cluster valves commonly are ball-drop valves as well. The ball seats in the cluster valves, however, are designed to catch and release a ball. That is, the ball seat has an initial ball-catch state where a ball will lodge against the seat. Once the ball is seated, hydraulic pressure applied above the ball will drive the valve sleeve downward to open the ports. Once the ports are opened, however, the seat will transition to a ball-pass state which releases the ball and allows it to travel through the valve. Once the ball exits the cluster valve, it will travel down the liner to actuate either another cluster valve or the static valve.
Thus, an operator may be able to fracture a number of clustered zones in a single stage. A ball deployed into the liner will travel through each cluster valve in the cluster and open them in succession. It then will land in the static valve at the bottom of the cluster, opening it and forcing fluid to be diverted out of the static valve and all the cluster valves associated with it. By fracturing multiple zones in a single stage, an operator may be able to complete fracturing of the well more quickly and efficiently.
As the number of cluster valves in a cluster is increased, however, ensuring that all valves in a cluster are fully opened may become more problematic. When the top cluster valve is opened, hydraulic fluid is able to begin flowing out of the valve before the lower cluster valves and the bottom static valve are opened. Any diversion of fluid out the top valve will cause the amount of fluid flowing down the liner and the hydraulic pressure in the liner to fluctuate. The next cluster valve opened will present additional paths for fluid to flow out of the liner. Thus, as a ball travels through a set of clustered valves, it may become progressively more difficult to adjust and control pumping of fluid into the liner so as to ensure that all valves in the cluster are opened completely.
The ability to selectively inject fluid into various zones in a well bore is important not only in fracturing, but also in other processes for stimulating hydrocarbon production. Aqueous acids such as hydrochloric acid may be injected into a formation to clean up the formation. Water or other fluids may be injected into a formation from a “stimulation” well to drive hydrocarbons toward a production well. The ability to selectively flow fluids out a series of valves may improve the efficiency and efficacy of those stimulation processes. Moreover, as in fracturing a well, an operator may prefer to stimulate one zone at a time or to stimulate clusters of zones simultaneously.
Additionally, as a liner incorporates more ball-drop valves, the valves may incorporate ball seats of progressively smaller sizes which can significantly restrict the flow of production fluids up through the liner. Thus, operators may prefer to drill out ball seats in ball-drop valves after a formation has been fractured or otherwise stimulated.
The statements in this section are intended to provide background information related to the invention disclosed and claimed herein. Such information may or may not constitute prior art. It will be appreciated from the foregoing, however, that there remains a need for new and improved sliding sleeve cluster stimulation valves and for new and improved methods for fracking or otherwise stimulating formations using sliding sleeve cluster valves. Such disadvantages and others inherent in the prior art are addressed by various aspects and embodiments of the subject invention.